Posterior pituitary Dr. Hana Alzamil. Hypothalamic control Posterior pituitary hormones ADH...
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ENDOCRINOLOGY
Posterior pituitary
Dr. Hana Alzamil
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PHYSIOLOGY OF POSTERIOR PITUITARY GLAND Hypothalamic control Posterior pituitary hormones
ADH Physiological functions Control of secretion
Osmotic stimuliNon-osmotic stimuli
Oxytocin Physiological functions Control of secretion
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POSTERIOR PITUITARY GLAND
Does not synthesize hormones
Consists of axon terminals of hypothalamic neurons
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HYPOTHALAMIC CONTROL OF PITUITARY SECRETIONS Secretions of the posterior pituitary are
controlled by Nervous signals from hypothalamus
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OXYTOCIN AND VASOPRESSIN ARE MANUFACTURED IN THE HYPOTHALAMUS, BUT RELEASED IN THE POSTERIOR PITUITARY
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Herring Body
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ANTIDIURETIC HORMONE
(vasopressin)
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SYNTHESIS OF ADH It is synthesized as pre-prohormone
and processed into a nonapeptide (nine amino acids).
ADH synthesized in the cell bodies of hypothalamic neurons(supraoptic nucleus)
ADH is stored in the neurohypophysis (posterior pituitary)—forms the most readily released ADH pool
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RECEPTORS OF ADH (VASOPRESSIN) There are 3 types of receptors for ADH:
V1A
V1B
V2
V1A receptors mediate vasoconstriction V1A receptors also found in the liver
glycogenolysis V1B receptors are unique to anterior pituitary
and mediate increased ACTH secretion V2 receptors are located in the principle cells
in distal convoluted tubule and collecting ducts in the kidneys
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MECHANISM OF ACTION OF ADH: ANTIDIURESIS
ADH binds to V2 receptors on the peritubular(serosal) surface of cells (principle cells) of the distal convoluted tubules and medullary collecting ducts.
Via adenylate cyclase/cAMP induces production and insertion of aquaporin2 into the luminal membrane and enhances permeability of cell to water.
Increased membrane permeability to water permits back diffusion of solute-free water, resulting in increased urine osmolality (concentrates urine).
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MECHANISM OF ACTION OF ADH
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THE SINGLE MOST IMPORTANT FUNCTION OF ADH IS TO
CONSERVE BODY WATER BY REDUCING URINE OUTPUT
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If plasma osmolality is directly increased by administration of solutes, only those solutes that do not freely or rapidly penetrate cell membranes, such as sodium, cause ADH release.
Conversely, substances that enter cells rapidly, such as urea, do not change osmotic equilibrium and thus do not stimulate ADH release.
ADH secretion is very sensitive to changes in osmolality.
Changes of 1-2% result in increased ADH secretion.
SECRETION OF ADH OSMOTIC STIMULI
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Hypovolemia is perceived by “pressure receptors” -- carotid and aortic baroreceptors, and stretch receptors in left atrium and pulmonary veins.
Normally, pressure receptors tonically inhibit ADH release.
Decrease in blood pressure induces ADH secretion by reducing input from pressure receptors.
The reduced neural input to baroreceptors relieves the source of tonic inhibition on hypothalamic cells that secrete ADH.
Sensitivity to baroreceptors is less than osmoreceptors– senses 5 to 10% change in volume
SECRETION OF ADHNON-OSMOTIC STIMULI
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FUNCTION OF ADH (VASOPRESSIN)
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REGULATION OF ADH
DehydrationADH released
OverhydrationADH inhibited
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Plasma Osmolarity stimulates both ADH release and thirst via
OSMORECEOPTORS
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Angiotensin II ADH
IncreasedBlood
Pressure
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OTHER STIMULI THAT AFFECT ADH SECRETION
Stimuli that increase ADH secretion:PainNauseaSurgical stressEmotional stress
Stimuli that decrease ADH secretion:Alcohol intake
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Receptors Osmoreceptors Baroreceptors
Location Anterolateral hypothalamus
Carotid sinus & aortic arch
Value Measured Plasma osmolality Circulating volume
ADH Release Stimulated By Activation of receptor Suppression of
receptor
Change Required for Action
1% above 280 mosm/kg 10-15% decrease
Resulting Amount of ADH Small Large
(vasoconstriction)
Override Other? no yes
TABLE SUMMARIZES THE MAJOR CHARACTERISTICS OF OSMORECEPTORS AND BARORECEPTORS
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CONTROL OF ADH RELEASE
Osmotic pressure: Osmoreceptor mediatedosmolality ADH secretionosmolality ADH secretion
Volume effects Baroreceptor mediated (vagus nerve)blood pressure ADH secretionblood pressure ADH secretion
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OXYTOCIN
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FUNCTION OF OXYTOCIN Breast-feeding
contracts the myoepithelial cells of the alveoli
(classic neuroendocrine reflex)
Childbirth (parturition) in late pregnancy, uterine smooth muscle
(myometrium) becomes sensitive to oxytocin
(positive feedback)
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BREAST FEEDING
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CHILDBIRTH
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Pitocin
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OTHER STIMULI THAT CONTROL RELEASE OF OXYTOCIN
In humans, oxytocin is thought to be released during hugging, touching, and orgasm in both sexes.
Release increased during stress Release inhibited by alcohol In males secretion increases at time of ejaculation (contraction of
smooth muscle of vas deferens)
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OXYTOCIN AND AUTISM Autistic group had significantly lower
plasma oxytocin levels than in the non-autism group
Elevated oxytocin was associated with higher scores on social and developmental measures for the non-autistic children
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SUMMARY OF POSTERIOR PITUITARY HORMONES ACTIONS
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